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<a name="Constraints"></a>
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Next: <a href="Standard-Names.html#Standard-Names" accesskey="n" rel="next">Standard Names</a>, Previous: <a href="Predicates.html#Predicates" accesskey="p" rel="prev">Predicates</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Operand-Constraints"></a>
<h3 class="section">17.8 Operand Constraints</h3>
<a name="index-operand-constraints"></a>
<a name="index-constraints"></a>

<p>Each <code>match_operand</code> in an instruction pattern can specify
constraints for the operands allowed.  The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
</p>
<p>Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have.  Constraints can also require two operands to match.
Side-effects aren&rsquo;t allowed in operands of inline <code>asm</code>, unless
&lsquo;<samp>&lt;</samp>&rsquo; or &lsquo;<samp>&gt;</samp>&rsquo; constraints are used, because there is no guarantee
that the side effects will happen exactly once in an instruction that can update
the addressing register.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Simple-Constraints.html#Simple-Constraints" accesskey="1">Simple Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Basic use of constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="2">Multi-Alternative</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">When an insn has two alternative constraint-patterns.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Class-Preferences.html#Class-Preferences" accesskey="3">Class Preferences</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Constraints guide which hard register to put things in.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Modifiers.html#Modifiers" accesskey="4">Modifiers</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">More precise control over effects of constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Machine-Constraints.html#Machine-Constraints" accesskey="5">Machine Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Existing constraints for some particular machines.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives" accesskey="6">Disable Insn Alternatives</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Disable insn alternatives using attributes.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Define-Constraints.html#Define-Constraints" accesskey="7">Define Constraints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to define machine-specific constraints.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="C-Constraint-Interface.html#C-Constraint-Interface" accesskey="8">C Constraint Interface</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to test constraints from C code.
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